OK, perhaps I was over-stepping the mark by saying you need a non-residency certificate to be legal in relation to owning a car.
The fact remains that any non-resident with a bank account in Spain must have a certificate of non-residency. Most people allow their banks to obtain this certificate and charge them for doing so.
Although there's no legal requirement for non-residents to have such a certificate when driving a Spanish registered car, there have been instances when the police have penalised drivers who have not been able to produce the certificate. The penalty can be challenged, and the challenge will succeed, but I'd argue that, as you need it anyway, why not carry a copy and save yourself any possible hassle with the police. And why allow the bank to charge you more than it would cost to get it yourself?